Neurons in the primary visual cortex exhibit response properties that are selective for various stimulus features, such as orientation preference, ocular dominance, and direction selectivity. These response properties are commonly arrayed in orderly columnar maps across the visual cortex, and their development is highly affected by manipulations of visual experience. Recent results have suggested that the development of direction selectivity may be unique in its early requirement for visual experience within the first two weeks following eye opening. Furthermore, in visually naove ferrets brief exposure to a drifting grating stimulus (motion training) is sufficient to drive the emergence of direction selectivity in a matter of hours. The experiments in this proposal are designed to determine which elements of the cortical circuit contribute to this rapid emergence of direction selectivity. Visual information from the retina reaches the cortex by way of LGN projections to layer 4, which in turn projects to layer 2/3, where the effects of motion training have been previously studied. The proposed experiments will first distinguish training-induced changes in geniculo- cortical synapses from intracortical mechanisms by examining the interocular transfer of motion training using in vivo two-photon calcium imaging. Subsequent experiments will probe the interactions within and between cortical layers that drive the emergence of direction selectivity. These experiments will use extracellular recordings in layer 4 and calcium imaging in layer 2/3 to compare the initial selectivity in each layer and the effects of motion training. A final set of experiments will use optogenetic tools to manipulate neural activity in layer 2/3 neurons during motion training. These experiments will test whether the presence and pattern of visually driven activity in layer 2/3 during training is essential for the emergence of direction selectivity. Taken together, these experiments will provide novel insights into the early events in visual system development that are critical for the proper maturation of direction perception. Understanding how experience with moving stimuli alters visual circuits to give rise to direction selectivity is crucial to understanding both this key feature of visual perception, and more broadly, the experience-dependent development of sensory systems.